This application is designed to study the process of endochondral ossification, with particular emphasis on the changes in gene expression that accompany chondrocyte hypertrophy and cartilage mineralization. The work will be carried out using a cell culture system, with chondrocytes derived from chick embryo sternae. The cephalic portion of the sternae yield chondrocytes which can be induced to undergo maturation and hypertrophy, whereas the caudal portion yields chondrocytes that are phenotypically stable. The experimental work is based on recent studies by the applicant showing that bone morphogenetic proteins (BMP2 and BMP4) induce expression of the genes for type X collagen and alkaline phosphatase that characterize chondrocyte hypertrophy, and at a later stage the expression of the matrix metalloproteinase MMP13 which may be required for modifying the cartilage architecture prior to mineralization. In the case of the type X collagen gene, a BMP-responsive region has been identified within its promoter that stimulates gene transcription, and whose activation is enhanced by ascorbate. The goal of the present application is to understand the mechanisms whereby chondrocyte hypertrophy and matrix mineralization may be regulated by the action of BMPs. Three specific aims are described to achieve this goal. 1) To define the transcriptional mechanism whereby BMPs induce changes in gene expression leading to chondrocyte hypertrophy using the type X collagen gene promoter. 2) To test the hypothesis that BMP activation of an intracellular signaling pathway involving its type IB receptor is essential for chondrocyte hypertrophy. 3) To examine the relationship between BMP-mediated MMP13 expression and matrix mineralization.